Carbonaceous materials are base materials essential for development of modern science and technologies and have been supplied and developed as energy sources for human beings. These materials are being actively studied together with nano carbon materials, i.e., fullerene compounds (1985), carbon nanotubes (1991), and recently, graphene compounds (2004), which have been discovered as the nano science develops. Especially, graphene is a monolayer carbon compound having a 2D structure and has superior characteristics such as a large surface area, high carrier mobility, and strong mechanical strength. Accordingly, graphene is highly expected to act as a material, which replaces the market for silicon-based electronic devices. However, graphene has a problem in view of application thereof because aggregation occurs among graphenes so that a dispersibility of the graphenes in a general solvent is significantly decreased.
As one of methods for overcoming the problem, a small nano-sized graphene quantum dot method has been researched and developed over recent years. A graphene quantum dot compound is a 0D material having a size of from about a few nanometers to about tens of nanometers. The graphene quantum dot compound is easily dispersed in various organic solvents and has a light emitting characteristic. Accordingly, the graphene quantum dot compound can be applied to bio imaging researches, light emitting devices, and photoelectronic devices.
A conventional light emitting device using a graphene quantum dot directly uses a graphene quantum dot or a mixed form of the graphene quantum dot with an inorganic nano-material, e.g., ZnO nano-particles.
In case of directly using a graphene quantum dot, quantum efficiency in a photo-luminescent spectrum (PL) is very low (2 to 22.4%). This result causes significant decrease of device efficiency in realizing the device.
In addition, there has been reported a white LED using a ZnO-graphene quantum dot hybrid type of a graphene quantum dot obtained by reacting graphene with ZnO nano-particles. In this case, however, when the graphene quantum dot device was realized, significantly low brightness of 798 cdm−2 was reported (Emissive ZnO-graphene quantum dots for white-light-emitting diodes, Nature Nanotechnology, 7, 465, 71, 2012).
As described above, the conventional light emitting device using a graphene quantum dot exhibits low light emitting efficiency as it uses a quantum dot having low quantum efficiency. In addition, when a device is manufactured by applying an organic material to an electron transport layer or a hole transport layer necessary for the device, high temperature deposition equipment should be used. Further, the organic material is not suitable for a flexible device due to its easy breaking property.